In an electrophotographic engine, a primary imaging member (PIM) such as a photoreceptive member, often referred to as a photoconductor, is initially uniformly charged by known means such as a grid controlled AC or DC corona charger, a roller charger, or other known means. An electrostatic latent image is then formed on the PIM by image-wise exposing the PIM to light, using known means such as laser scanners, LED arrays, or optical exposure. The electrostatic latent image is then converted into a visible image by bringing the PIM into close proximity with a development station containing a developer. The developer may contain toner particles that contain a colorant and are known as marking particles. Alternatively, the toner particles may lack colorant and be known as clear toner. Some typical present day toner particles have a volume-weighted diameter of between 4 μm and 9 μm. In addition, some toner particles often are coated with nanometer-size clusters of particulate addenda such as SiO2, TiO2, etc. Such addenda improve flow and transfer by reducing adhesion and also help to control the charge of the toner particles. The developer frequently contains carrier particles that are known to be used in so-called two component developers. Such developers lack solvent such as various hydrocarbons or silicones and are generally referred to as dry developers and the process of developing the toner image referred to as dry electrophotographic development. The carrier particles are often magnetic particles and serve to transport the toner particles using magnets in the development station. The carrier particles also serve to impart a controlled charge on the toner particles through triboelectrification. This charge allows the particles to be attracted to and thus develop the electrostatic latent image. The charge also allows the toner particles to be transferred to another substrate such as a transfer intermediate member or a receiver such as paper.
After development, the visible or toner image is transferred to a receiver. This is generally accomplished by subjecting the electrically charged toner particles to an electrostatic field that urges the particles towards the receiver while bringing the receiver into contact with the toner particles.
In many instances, the toner image is transferred directly to a receiver such as paper. The image is then permanently fixed to the receiver. This is generally accomplished by subjecting the image-bearing receiver to a combination of heat and pressure, although alternative methods such as employing the use of microwave or RF electromagnetic radiation, radiant heat, solvent vapors, etc. are occasionally employed. After transfer, the PIM is cleaned and made ready for subsequent imaging.
To produce color prints, electrostatic latent images corresponding to specific color information are first produced on the PIM. These generally correspond to the subtractive primary colors, cyan, magenta, yellow, and black. The separate electrostatic images are made visible by bringing the PIM into close proximity to a development station containing toner of the appropriate color. The images are then transferred to a receiver, in register, generally by pressing the receiver in contact with the PIM under an applied electrostatic field repeatedly until each of the subtractive primary toner images has been transferred. The image is then fixed to the receiver, generally upon application of heat and pressure.
In some instances it is preferable to first transfer the toner image or images to one or more transfer intermediate members, especially compliant transfer intermediate members. In one embodiment of such, each color image is transferred to a separate intermediate member. The images are then transferred in register, sequentially, to the receiver. In an alternative embodiment, the images are transferred in register to the intermediate transfer member (ITM) and then the registered toner image is transferred to the receiver. In both cases, the toner transfer is accomplished by first pressing the ITM into contact with the PIM while applying an electrostatic field to urge the toner to the ITM. The receiver is then pressed against the ITM and an electrostatic field exerted to urge the toner image from the ITM to the receiver.
In order to maintain image quality such as low levels of granularity and high resolution, it is desirable to use small toner particles. For dry electrophotographic developers, small toner particles typically have diameters between 5 μm and 9 μm. Unless otherwise noted, the term toner diameter refers to the volume-weighted diameter of toner, as measured with a Coulter Multisizer or comparable device. Smaller toner particles are difficult to transfer and have restricted flow properties. Larger toner particles create high granularity and reduce resolution.
It is possible to produce desirable graphic arts effects using raised letters without degrading image quality by using large clear toner particles. However, the use of clear toner would require that the electrophotographic engine being used have more than the four development stations required for a regular subtractive primary color printer and if one of the primary color stations were removed and large clear toner substituted in that particular station that would degrade the ability of the printer to produce high quality color prints spanning the color gamut.
It is clear that a new process that does not rely on the presence of large clear toner is needed to produce raised print with compact printers, such as an engine containing four or fewer development stations. This invention discloses a method and apparatus capable of meeting these needs.